Space-charge effects can limit the performance of ion mobility spectrometers and mass spectrometers in many ways. One of the most common limitations is the ability to accumulate large populations of ions in an ion trap prior to ejection or release of the ions into a downstream analyser. Space-charge effects can be mitigated in ion traps by providing the ion trap with relatively large dimensions so as to provide a relatively large ion trapping volume. However, in order to simultaneously eject ions from such an ion trap the ions must be ejected in a packet or pulse that is relatively wide. The ion packet must then be focussed down in width so as to be transmitted through the relatively narrow differential pumping apertures that are arranged between regions of different pressures. However, this focussing compresses and concentrates the ion packet and hence reintroduces the problem of space-charge effects.
It is desired to provide an improved method of mass spectrometry or ion mobility spectrometry, and an improved mass spectrometer or ion mobility spectrometer.